Can anyone give a explaination of "image force" and how it affect the work function of metal? Thanks,

The image force is the interaction due to the polarization of the conducting electrodes (i.e. of the conducting tip and the substrate) by the charged atoms of the sample. This is important for any tip-surface (or just surface) setup containing conducting materials e.g. interaction of a conducting tip with an insulating surface or studying the properties of an insulating thin film on top of a metal substrate.

The potential on conducting electrodes is maintained by external sources (i.e. by the ``battery''). From the point of view of classical electrostatics the polarization of the conductors by external charges is caused by the additional potential on the conductors due to the charges. This extra potential is compensated by a charge flow from one electrode to another to keep the potential on the conductors fixed. This work is done by the battery. As a result, there will be some distribution of the net charge on the surfaces of conductors induced by the point charges situated in the free space between them. The net charge on each conducting electrode would interact with the total charge on other conductors and with the point charges. Hereafter this interaction is called the image interaction.